cleanup: Static_binary_string need not take non-const double parameter

Convert the parameter to const as the function won't modify the pointer
value.

sql/sql_string.cc

@@ -767,10 +767,10 @@ void Static_binary_string::qs_append(double d)
                        NULL);
 }
 
-void Static_binary_string::qs_append(double *d)
+void Static_binary_string::qs_append(const double *d)
 {
   double ld;
-  float8get(ld, (char*) d);
+  float8get(ld, (const char*) d);
   qs_append(ld);
 }
 

sql/sql_string.h

@@ -345,7 +345,7 @@ class Static_binary_string : public Sql_alloc
   void qs_append(const char *str, size_t len);
   void qs_append_hex(const char *str, uint32 len);
   void qs_append(double d);
-  void qs_append(double *d);
+  void qs_append(const double *d);
   inline void qs_append(const char c)
   {
      Ptr[str_length]= c;